Magnetic nanoparticles and magnetic nanobeads (MNBs), i.e., superparamagnetic nanoparticles embedded in a polymeric matrix, represent a widely spread tool in modern biomedical technologies. In particular their utilization in different biosensing schemes has been explored in the last few years. The long-range interaction between magnetic nanoparticles and an external magnetic field enables easy manipulation and sensitive detection. The main advantages offered by biosensing approaches utilizing magnetic fields and magnetic carriers are that biological media have an intrinsic low magnetic susceptibility and the magnetic interactions are generally not affected by surface charges, pH values, ionic concentrations or temperature. In addition, the realization of a microfluidic device based on magnetic carriers to capture, sort and detect target analytes in biological media is particularly appealing, due to the potential low-cost, simplicity of the device and high sensitivity achievable.
U.S. Pat. No. 7,639,359 discloses a method for detection of analytes by measuring single particle dynamics of bio-functionalized magnetic nanoparticles. The method in U.S. Pat. No. 7,639,359 comprises applying linearly polarized light to a suspension of bio-functionalized nanoparticles subjected to an oscillating uniaxial magnetic field and subsequently measure how the polarization of the light rotates when passing through the magnetic nanoparticles suspension. In this case the signal monitored is in the first harmonic with respect to the magnetic field excitation.
One disadvantage with the method disclosed in U.S. Pat. No. 7,639,359 is that it is complex, it relies on a very small signal due to light polarization rotation and consequently the setup is costly in that it requires optical elements such as aligned polarizers in order to ensure that the rotation of the light polarization is correctly measured. Another disadvantage with the method disclosed in U.S. Pat. No. 7,639,359 is that it requires single domain magnetic nanoparticles in order to work that these are generally not efficient light scatters.
US20120003750 discloses a method for detection of an analyte by measuring the dynamics of analyte-driven cluster formation of bio-functionalized superparamagnetic particles. The method in US20120003750 comprises allowing the superparamagnetic particles in a suspension to form analyte-driven clusters in the presence of a rotating magnetic field and thereafter measure the intensity and amplitude of the scattered light at higher harmonics of the driving frequency.
One disadvantage of the method disclosed in US20120003750 is the requirement of electromagnets creating a rotating in-plane magnetic field. Having electromagnets generating a rotating field will take up more space thus limiting practical implications such down-sizing of the biosensing setup or integration of the setup into other systems. In addition the signal due to scattered light from magnetic beads at a large angle from the incident light is extremely low, especially for particles of few hundreds nm of diameter, and a large area and very sensitive photodetector or photomultiplier is needed.